1. Field of the Invention
This invention relates to improvements in spectacles especially of the type having plastic frame fronts and plastic lenses.
2. Discussion of the Prior Art
At present, plastic ophthalmic lenses, e.g. those made of allyldiglycolcarbonate, polymethyl-methacrylate or polycarbonates, are mounted in metal or plastic spectacle frames according to techniques developed more specifically for the mounting of lenses made of silicate glasses.
In the case of metal frames, rims holding the lenses are tightened by one or more screws. In plastic frames, a cool lens is inserted in a heated, expanded frame and held thereinplace by forces resulting from shrinkage of the frame rims after the frame is cooled.
These mounting techniques apply relatively high circumferential clamping pressures to the lenses which can be withstood by glass lenses but not so with plastic lenses. Plastic lenses which are relatively flexible can and commonly become distorted by conventional lens-mounting techniques with the result of unwanted and uncontrolled changes in their optical performance after they have been mounted.
Attempts to reduce the distortion of plastic lenses by reducing clamping pressures below those normally used are thwarted by temperature changes in wearing environments which can further loosen the lenses to the extent that they can rattle in the frame and may even fall out.
The aforesaid and related problems of mounting plastic ophthalmic lenses are overcome according to the present invention by abandoning conventional mechanical clamping of plastic lenses in spectacle frames. Strong and permanent lens support under all normal wearing conditions and temperatures is accomplished by a unique selection of compatible lens and frame materials wherein frames having the same or lower tensile elastic modulus and/or the same coefficient of thermal expansion as the lens materials are used.